(1) Field of the Invention
This invention relates to an electrically steerable phased array antenna system. It is intended for use in many areas, for example telecommunications and radar, but finds particular application in cellular mobile radio networks, commonly referred to as mobile telephone networks. More specifically, but without limitation, the antenna system of the invention may be used with second generation (2G) mobile telephone networks such as the GSM, CDMA (IS95), D-AMPS (IS136) and PCS systems, and third generation (3G) mobile telephone networks such as the Universal Mobile Telephone System (UMTS), and other cellular radio systems.
(2) Description of the Art
Cellular mobile radio networks which use phased array antennas are known: such an antenna comprises an array of individual antenna elements (usually eight or more) such as dipoles or patches. The antenna has a radiation pattern consisting of a main lobe and sidelobes. The centre of the main lobe is the antenna's direction of maximum sensitivity, i.e. the direction of its main radiation beam. It is a well known property of a phased array antenna that if signals received by antenna elements are delayed by a delay which varies linearly with element distance from an edge of the array, then the antenna main radiation beam is steered towards the direction of increasing delay. The angle between main radiation beam centres corresponding to zero and non-zero variation in delay, i.e. the angle of steer, depends on the rate of change of delay with distance across the array.
Delay may be implemented equivalently by changing signal phase, hence the expression phased array. The direction of the main beam of an antenna pattern can therefore be altered by adjusting the phase relationship between signals fed to different antenna elements. This allows the beam to be steered to modify the coverage area of the antenna.
Operators of phased array antennas in cellular mobile radio networks have a requirement to adjust their antennas' vertical radiation pattern, i.e. the pattern's cross-section in the vertical plane. This is necessary to alter the vertical angle of the antenna's main beam, also known as the “tilt”, in order to adjust the ground coverage area of the antenna. Such adjustment may be required, for example, to compensate for change in cellular network structure or number of base stations or antennas. Adjustment of antenna angle of tilt is known both mechanically and electrically, and both individually and in combination.
Control of an antenna's angle of electrical tilt is disclosed in International Patent Application Nos. WO 03/036756, WO 03/036759, WO 03/043127, WO 2004/088790 and WO 2004/102739. Of these, WO 2004/102739 in particular discloses control of electrical tilt by varying a phase difference between a pair of signals: a signal splitting and recombining network forms a set of different vectorial combinations of these signals with appropriate phasing for input to respective antenna elements.
However, WO 2004/102739 suffers from the disadvantage that it employs track cross-overs, i.e. circuit regions providing for one signal to cross another. Track crossovers require either a three-dimensional circuit (multilayer design) or a two-dimensional circuit incorporating track cross-over networks. The three dimensional approach increases circuit size and bulk: it requires a large radome and results in high cost. A planar printed circuit approach can reduce circuit size and cost, but the resulting need to employ cross-over networks significantly increases signal losses and reduces the gain of the antenna. Use of a significant number of hybrids and cross-over networks also reduces the bandwidth over which the antenna gain beam pattern can be maintained.